‘It’s more severe with twins’

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Half a million pregnant women are troubled by morning sickness each year in the UK, but only 1 in 100 of them will be affected so badly that they can’t keep anything down and become worryingly dehydrated (hyperemesis gravidarum).

Morning sickness is more common, and often more severe, in multiple pregnancies, such as twins, but it is still not fully understood. One theory is that changing levels of hormones act centrally on the brain to heighten an evolutionary adaptive response designed to help prevent women consuming substances that may be harmful.